1. Field of the Invention
The invention relates to the field of transversal filters employing charge transfer devices, particularly those filters employing split electrode structures or capacitive sensing.
2. Prior Art
It is well-known in the transversal filter art to fabricate such filters with analog shift registers. Most often charge transfer devices such as bucket-brigade devices (BBD) or charge-coupled devices (CCD) are employed.
There are numerous known means for obtaining different tap weights at the stages of charge transfer devices in order to provide a predetermined filtering characteristic. Capactive and resistive tap weights are used in addition to binary weighting and other weighting means. For an example of BBD filters employing both capacitive weighting and resistive weighting, see U.S. Pat. No. 3,546,490, FIGS. 15 and 17.
Split electrode structures are employed in transversal filters to provide fixed tap weights. These filters are particularly suitable for integrated circuits since the split electrodes are easily defined by a photolithographic masking step. Split electrode filters provide high accuracy and moreover, may be fabricated at a relatively low cost. For a general discussion of transversal filters including filters employing split electrode structure, see Charge Transfer Devices by Sequin & Tompsett, published by Academic Press, Inc. 1975 beginning at page 216.
The charge transfer, split electrode transversal filters typically include a plurality of parallel clocked electrodes or gates formed over a channel. The gates associated with one clocking signal are split in a predetermined manner to provide the desired filter characteristic. In general, as charge is transferred under these split gates or electrodes its effect on the clock lines is sensed. The output signal is generally much smaller than the clocking signal, making the output circuitry relatively complex and reducing the dynamic range of the output signal. (A prior art split electrode filter will be discussed in conjunction with FIG. 1). For a general discussion of these problems see "The Design & Operation of Practical Charge-Transfer Transversal Filters", IEEE Journal of Solid-State Circuits, Volume. SC11, No. 1, February 1976 beginning at page 65, (See the discussion on page 68 beginning under the heading "Clocking & Signal Recovery".)